TalkTalk’s chief executive has apologised to its customers after a “significant and sustained cyber attack” on its website left millions of people potentially at risk of having had their credit card and bank details stolen.

Dido Harding defended the firm for not revealing the security breach until last night, despite it taking place on Wednesday morning.

She admitted the firm does not yet know how many of its four million customers had been affected by what is the third in a spate of cyber attacks affecting TalkTalk customers in the last eight months.

A major TalkTalk contact centre, operated by HGS, is based in Preston at Tulketh Mill.

More than 400 staff are employed to deal with the telecoms firm’s customers and other companies.

In August TalkTalk revealed its mobile sales site was hit by a “sophisticated and co-ordinated cyber attack” in which personal data was breached by criminals.

I really appreciate the frustration and the worry and the concern that this causes customers - I am a customer myself - and I am very sorry for that

TalkTalk chief exec

And in February TalkTalk customers were warned about scammers who managed to steal thousands of account numbers and names from the company’s computers.

Ms Harding told ITV’s Good Morning Britain the three attacks were “completely unrelated”, adding: “We moved as fast as we possibly can, on Wednesday lunchtime all we knew was that our website was running slowly and that we had the indications of a hacker trying to attack us.

“I can’t even tell you today exactly how many customers have been affected. We have tried to come public as fast as we can once we have got a reasonable idea of what potential data has been lost.

“I really appreciate the frustration and the worry and the concern that this causes customers - I am a customer myself - and I am very sorry for that. We are rushing to try and get that information to our customers as fast as we possibly can.”

She added: “This is a crime, a criminal has attacked TalkTalk systems and we are not the only ones, whether it is the US government, Apple, a whole host of companies. Cyber crime is something we all need to get better at defending ourselves against.”

A spokesman for global outsourcing company HGS did not want to comment today.

Customer advice

TalkTalk is warning customers to pay attention to their bank accounts for signs of fraudulent activity.

The information stolen could be sufficient for fraudulent payments to be made, the company says it has contacted banks to warn them of this.

Users should check their credit report, and TalkTalk recommends using the three main credit agencies (Call Credit, Experian and Equifax) to do so.

The company says it is “looking to organise a year’s free credit monitoring for all of our customers and will be in touch on this in due course”.